1. Field of the Invention
This invention relates to the field of microchip assembly and in particular to connecting a die-substrate having a solder ball array to a substrate having a solder land array.
2. Discussion of Related Art
The ever-increasing density improvements in silicon can be more fully exploited with corresponding wiring density improvements in chip carriers. Challenges to increased wire density have been met with ball grid array packaging designs. However, as the number of balls per unit area has increased, such as with C4 flip-chip designs, so have the routing problems. FIG. 1A illustrates a via-to-bonding pad design that is in the shape of a xe2x80x9cdog bonexe2x80x9d. The dog bone design has the solder land (bonding pad) distinctly separate from the via and connected with routing. Along with the need to route each bonding pad (pad) to each via in the dog bone design, there is also the increased difficulty of routing other circuitry around the xe2x80x9cdog bonesxe2x80x9d. These routing problems can be reduced by designing with a via centered within the bonding pad (xe2x80x9cvia in padxe2x80x9d or VIP). FIG. 1B illustrates the pad centered on the via. VIP can free a number of routing channels and reduce the layer count but will also increase bridging caused by voids formed within the soldered balls. Solder ball bridging with the VIP design has been seen in up to 50% of product assembled in a high volume factory.
FIGS. 1C-1E illustrate how bridging problems occur with via in pad designs on a substrate such as a printed circuit board (PCB). Bridging problems are a result of gas expansion from one or more vias into one or more mating solder balls of a die-substrate at an elevated temperature point in assembly. FIG. 1C illustrates how the problem begins when the solder ball, centered over the pad and therefore over the via hole, contacts the pad/via, seals the via opening along with any gas within. FIG. 1D illustrates how, during a reflow operation, the sealed via gas is unable to vent and expands into the solder ball as the solder begins to liquefy. FIG. 1E illustrates where, once the solder ball approaches a liquid state, the via gas can become encapsulated within the solder and continue to expand as temperature is increased. Via gas expansion has the effect of expanding the solder ball as if blowing up a balloon. The solder ball expansion can continue until it contacts an adjacent solder ball causing bridging to occur. The solder bridging problems discussed generally involve the use of eutectic solder in the solder ball. Eutectic solder will flow as a liquid during the reflow heating process.
FIG. 1F illustrates another bridging problem where an adjacent (non-connected) solder ball flows to contact a via. This can occur when solder ball spacing is too close along with solder mask that is very narrow or missing. To minimize this condition, solder ball spacing can be increased or, as illustrated in FIG. 1G, caps placed over the vias on the substrate side contacted by the solder balls (primary side). These caps are placed on the substrate with solder mask material and constitute an additional process step. An increase in solder ball spacing can reduce solder ball density overall.